css source map
from: http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/
启用Source map:
@ sourceMappingURL=/path/to/file.css.map 可能需要添加
//
//# sourceMappingURL=/path/to/file.js.map
X-SourceMap: /path/to/file.js.map
chrome 启用 enable source map (默认启用)